1. Field of the Invention
The invention relates to a method and apparatus for measuring the size of objects on a moving conveyor. More specifically, the invention relates to a method and apparatus for measuring the size of objects on a conveyor using a photodiode light curtain to measure the height of the objects together with a two laser rangefinders which employ light detection cameras and pulse tachometers to measure the length and width of the objects on the conveyor.
2. Description of the Related Art
Objects traditionally have been measured by hand with ruled straight-edges and tape measures. However, in a conveyor environment, in which the objects on the conveyor are moving (i.e., excess of 100 objects per minute passing through a particular area on the conveyor), this is an unreasonable method for determining the size of the objects.
Unger et al. (U.S. Pat. No. 3,436,968 and U.S. Pat. No. 3,588,480) describes a dual light curtain system with one set of beams positioned horizontally with respect to the conveyor, and another set of beams positioned vertically with respect to the conveyor. Such an arrangement of a two-dimensional light curtain requires a break in the surface of the conveyor, due to the photodetectors on the bottom edge of the light curtain that interfere with the conveyor. The Unger system is undesirable due to the needed break in the surface of the conveyor, which is not always available in existing conveyor systems. Additionally, the Unger system requires carton orientation alignment prior to the measurement by the two-dimensional light curtain.
Henderson et al. (U.S. Pat. No. 3,513,444) describes a dual light curtain system which accumulates the sum of volume slices as the object is conveyed. This system can only measure the volume, and requires carton orientation alignment, as well as requiring a break in the surface of the conveyor.
Claesson et al. (U.S. Pat. No. 4,773,029) describes the use of two light curtains, one with horizontal beams and one with vertical beams, to measure objects on a conveyor. Like the above-mentioned system, the Claesson system also requires a break in the surface of the conveyor.
Hayashi et al. (U.S. Pat. No. 4,905,512) describes a method similar to the dual light curtain system of Claesson, except that in the place of an array of sensors, a single sensor is moved back and forth. The time required to move the sensor back and forth strongly limits the speed at which objects can be moved on the conveyor. This makes the Hayashi system inappropriate for high speed object measurements. The Hayashi system is appropriate for measuring long, continuous materials that change little in their respective lateral dimensions over time, such as extruded dough in a baking process.
Stringer et al. (U.S. Pat. No. 5,105,392) describes several measurement schemes, only one of which measures objects in motion. The in-motion scheme uses multiple ultrasonic sensors to sense the top, sides and back of an object when the front of the objects hits a cross-conveyor photo-eye. The use of ultrasonics carries with it some difficulties, such as the changes in the speed of sound due to temperature and air pressure, the relatively slow repetition rate due to the travel time of sound, and the large cone of solid angle sampled by the sensor, which makes it difficult to ascertain which part of an object is being measured. The Stringer system also requires that the object be orientationally aligned.
Stringer et al. (U.S. Pat. No. 5,220,536) describes a light curtain with vertical beams to obtain width information and ultrasonic sensors to get the height information. This approach suffers from the temperature and pressure sensitivity and repetition rate limitations of ultrasonics, as well as the vertical light beams requiring a break in the surface of the conveyor.
Jenseen et al. (U.S. Pat. No. 5,331,118) describes a system similar to the dual light curtain approaches described above, but utilizes barcode scanners and an array of barcodes in place of the light curtains. This approach suffers from the added complexity associated with the non-orthogonal geometry of the measuring scheme, as well as requiring a break in the surface of the conveyor.
Sjodin et at. (U.S. Pat. No. 4,179,707) describes a measuring system for substantially parallel objects on a conveyor using a light source, a background area for receiving light reflected of the objects, and a camera for receiving the light reflected off the background area. This system suffers from the need to have the objects substantially aligned prior to the measurement process, as well as requiring a background medium for the camera to properly receive the reflected light therefrom.
Dreyfus et al. (U.S. Pat. No. 4,226,536) describes a measuring system for helicopter rotor blades, using both a moving light transmitting source and a moving light receiving source. This system suffers from the need to perform advanced calculations related to the exact location of the two light sources with respect to time, as well as the need to maintain moving components in the system, which typically are more prone to failure than stationary components.